A spark plug is installed in an internal combustion engine (engine) or the like and is used for igniting an air-fuel mixture or the like in a combustion chamber. In general, a spark plug includes an insulator having an axial hole extending along the axis thereof, a center electrode inserted into the axial hole at a front end side thereof, a metallic shell disposed around the insulator, and a ground electrode fixed at a front end portion of the metallic shell. A spark discharge gap is formed between a front end portion of the ground electrode and a front end portion of the center electrode. When high voltage is applied to the center electrode (spark discharge gap), spark discharge occurs at the spark discharge gap, whereby an air-fuel mixture or the like is ignited.
When the insulator has insufficient insulation performance (dielectric strength), the dielectric breakdown of the insulator may occur due to the application of a high voltage to the center electrode. In this case, the discharge penetrating the insulator (so-called penetration discharge) may occur between the center electrode and the metallic shell. In a state that the penetration discharge may occur, the application of high voltage to the center electrode (spark discharge gap) may not result in the normal spark discharge in the spark discharge gap.
In view of this, in a process of manufacturing a spark plug, the insulation performance of the insulator is inspected. A method of inspecting the insulation performance is disclosed in, for example, JP-A-2012-185963. The method according to JP-A-2012-185963 produces a state in which the spark discharge does not occur in the spark discharge gap by the high-pressure gas. After that, high voltage is applied to the center electrode. Then, based on the waveform of the applied voltage, the insulation performance is inspected.
However, the discharge that may occur when high voltage is applied to the center electrode is not limited to the penetration discharge. The discharge may also occur, due to the spreading of the electric field across the surface of the insulator between the front end portion of the center electrode and the metallic shell. This kind of the discharge is so-called flashover. At least a part of the route of this discharge includes a front end surface of the insulator. Here, even though the flashover occurs, the dielectric breakdown of the insulator is not caused, which is different from the penetration discharge, and there is no particular problem in the insulation performance of the insulator. However, no particular difference is found between the waveform of the applied voltage in the occurrence of flashover and the waveform of the applied voltage in the occurrence of the penetration discharge. Thus, in the method according to JP-A-2012-185963, it is impossible to determine which one of the flashover and the penetration discharge has occurred. Therefore, a product in which flashover has occurred upon voltage application, but which actually has no problem with the insulation performance, may be treated as a defective product. Thus, this method may deteriorate the yield.